A Positive Airway Pressure (PAP) device is used in the treatment of sleep related breathing disorders such as Obstructive Sleep Apnea (OSA). A typical device consists of a flow generator which delivers pressurized air to a patient via an air delivery hose connected to a face mask. At its simplest level, the flow generator consists of a blower that can deliver a prescribed pressure (up to 30 cmH2O) to the patient, as set by a clinician.
High end PAP devices claim to be more effective in the treatment of OSA because they can monitor the air flow delivered to the patient and derive a measure of the effectiveness of the treatment pressure. Using this information, the quality of treatment can be improved and reported back to a clinician for review. In an auto-titrating device (APAP), the delivered pressure can be continually adjusted to the minimum required pressure for effective treatment.
Traditional techniques used to measure flow in a PAP device involve inserting a flow sensor in the air path. The sensor may be of a Venturi type, which measures a pressure drop across a section of the air path, usually across some known pneumatic resistance. Another sensing technique is the thermal mass flow sensor, which allows the air to pass over a heated element with the temperature being measured downstream of the element. These techniques are accurate yet add extra costs to the PAP device due to the sensing hardware.
Several patents have been granted on devices for treating disordered breathing that are capable of operation with traditional flow sensors. One example is U.S. Pat. No. 5,443,061 to Champain, et al., titled “Apparatus for providing a breathing gas with an overpressure and process of controlling such apparatus installation.” Champain uses a piezoelectric pressure sensor for detecting airflow fluctuations between a turbine and a mask. The output of the sensor is provided to a turbine controller, which adjusts the turbine accordingly.
Other examples of relevant prior patents include U.S. Pat. Nos. 5,740,795 and 6,237,593, both to Brydon, both having the same assignee as the present invention and both teaching “Estimation of flow and detection of breathing in CPAP treatment” (collectively “Brydon”). Brydon states that the speed of a blower motor is controlled by a feedback loop in which actual motor speed is measured and an error signal is generated to increase or decrease drive to the motor or other regulating device, thus maintaining a constant motor speed ('795 patent, col. 2, lines. 55-65). The structure for performing the speed control includes a motor controller which issues a control signal to control the motor speed. A speed feedback signal is inputted into the motor controller which provides the signal upon which speed regulation is based.
Brydon also states that signals can be derived from motor speed and power measurements and bear a non-linear relationship to the actual volumetric flow. These signals may be linearized using empirically determined pressure/flow/speed characteristics of the turbine system to give a volumetric measure of patient respiration (flow) ('795 patent, col. 3, lines. 5-15). Regarding power measurements, Brydon states that current alone is typically a sufficient indicator of motor power. To measure current, Brydon teaches a current sensing resistor and measurements of the voltage drop across the resistor. The voltage is sensed and provided to a differential amplifier, whose output is a signal representative of motor current (and power). The signal is then provided to a low-pass filter circuit that removes high frequency electrical noise, providing the average or steady state component of the signal. The signal is then extended through a high-pass filter to remove non-respiratory components and then applied to a single-dimension linearization element, the function of which is derived from empirically determined pressure/flow/speed characteristics of the turbine, tube and mask system. The output of the linearization element is a linearized flow signal.
The problem with the Brydon approach is that motor current is very noisy. In FIG. 1, the top trace 2 is mask pressure, while the bottom trace 4 is the motor current. For a large pressure step, the current trace indicates significant noise because of the summation of switching currents through motor drive MOSFETs that are usually used. It can be appreciated that for small pressure or flow perturbations, the signal noise becomes even more significant.
The flow information read from auto-titrating (APAP) and high-end constant pressure (CPAP) PAP devices does not necessarily require the accuracy provided by hardware flow sensors. Therefore, a flow estimator would provide an alternative, low-cost method to provide flow data, usable where cost requirements prohibit the use of sensors.